This is something that's been discussed time and again, even by Bill Brown and Jim Deshaies on the air, but FanGraphs' recent addition of Splits and their series of articles focusing on them called it back to mind, and I think now--following his wretched 2009 campaign as a RHB--is a good time to re-visit the issue.
Lance Berkman has a very exaggerated platoon split. As a switch hitter, when he faces lefties, he bats right-handed. Unfortunately, for a first baseman, and particularly compared to his skill as a left-handed batter, he's not very good at it.
2009 was a particularly poor year from the right-handed side. Berkman only batted for .702 OPS, as compared to his .982 line as a LHB. His strikeout rate was higher, his walk rate less than half, and his isolated power plummeted.
Some of this was due to poor luck on balls in play, yet it's hard to ignore how the switch saps his power and ruins his usually-excellent plate discipline. Furthermore, his career splits are not a lot better; 1.020 OPS as a LHB, and .793 as a RHB.
Who knows? His platoon split might be even worse as a LHB instead of a switch-hitter. There are also sentimental issues involved, as Berkman was taught by his father from childhood to switch hit, and he holds or is challenging a number of records as a switch hitter.
Still, it's hard to believe that his left-handed platoon split could be so exaggerated as to make up a career .227 OPS difference. If I were the Puma, I'd think about facing those lefties from the left-handed side and going with my strength.